In a wireless packet-switched data network employing Orthogonal Frequency Division Multiplexing (OFDM), modulated symbols are constructed from a data packet and inserted into a resource block enclosed by a rectangular area in the time-frequency domain. The resource block is allocated to the user of the data packet and transmitted to the user over the air. The resource block includes consecutive sub-carriers in the frequency domain and consecutive OFDM symbols in the time domain. For example, in LTE (long-term evolution), a resource block includes 12 consecutive sub-carriers in the frequency domain and 14 consecutive OFDM symbols in the time domain (7 symbols per slot). Each element of the resource block represents a basic unit in which a complex-valued symbol can be transmitted.
The user must estimate the channel over which the resource block is transmitted in order to coherently demodulate the symbols included in the resource block. Known reference symbols, commonly referred to as pilot symbols, are also transmitted in the resource block to enable channel estimation at the user. The known reference symbols are dedicated to the user for certain transmission modes meaning only that user can process the reference symbols. When dedicated reference symbols are used, the same transmission methods used for the data symbols are also used for the known reference symbols. For example, when preceding is used with multiple transmit antennas in combination with dedicated reference symbols, the precoding applied to the data symbols is also applied to the known reference symbols. Various cases that use dedicated reference symbols for a particular user include scenarios where pre-coding or beam directing and/or beam forming is used in MIMO (multiple-input multiple-output) systems. The user allocated the resource block uses the reference symbols included in the resource block to estimate the response of the channel and demodulates the data symbols included in the resource block based on the channel estimate.
The response of the wireless channel in an OFDM system is a slow-varying, two-dimensional function of time and frequency. To accurately estimate this function, the density and placement of the known reference symbols must be properly designed. In conventional OFDM systems, a resource block is a self-contained unit where both data and reference symbols are confined to the same resource block. The resource block is dedicated to the user for which the data packet is intended. As such, a user conventionally processes symbols extracted only from resource blocks allocated to that user, including reference symbols for channel estimation.
Better channel estimation performance can be achieved when the reference symbols are placed along the boundary (e.g., at the corners) of the resource block. This way, the entire part of the channel response can be interpolated from the observations at the boundary reference symbol locations instead of being extrapolated from only observations at interior reference symbol locations. However, many practical considerations limit the amount of reference symbol boundary allocations that can be made within a particular resource block. For example, reference signal sequences are commonly used to carry device identities such as cell identities. Restricting the reference symbols to border locations along or near the periphery of the resource block can severely limit the number of unique patterns available to identify multiple devices. In addition, MIMO advanced antenna systems employ spatial multiplexing so that different streams of data can be transmitted using the same resource block. However, the corner locations of the resource block can only be assigned to one of the antennas. Otherwise, severe interference can occur. The remaining antennas thus experience performance loss because the channel response is estimated by extrapolation from observations at interior reference symbol locations instead of interpolation from border observations.